Recently, storage batteries such as lithium ion batteries that may be repeatedly charged or discharged have been used in various fields such as portable devices, automobiles, etc. In order to improve the usability of devices using a storage battery, it is important to know how long the device will be operable. Thus, most devices using a storage battery have a function of calculating and displaying a residual capacity of the storage battery.
As typical methods of estimating a residual capacity of a storage battery, there are an open circuit voltage (OCV) measurement method and a Coulomb counting method. The OCV measurement method is a method of measuring an OCV of a storage battery in advance and creating a table of the measured OCV. According to the OCV measurement method, it is possible to obtain an accurate value during normal use. The Coulomb counting method is a method of estimating an amount of internal charges by frequently monitoring an inflow/outflow of electric charges to and from a storage battery. The Coulomb counting method is resistant to a load fluctuation such as an inrush current, and thus obtains a stable residual capacity.
However, the Coulomb counting method has shortcomings in that errors may be easily accumulated due to current integration.
Meanwhile, the OCV measurement method has the following problems. First, when a load greatly fluctuates within a short time, a calculation value is likely to fluctuate. Further, the OCV measurement method is based on a premise that a sufficient time should pass after charging and discharging and a storage battery should be stable. In addition, in a state where charging and discharging are continuously performed, a fluctuation in an open circuit voltage of a storage battery is not considered. In particular, a fluctuation of an open circuit voltage is increased at low temperatures, and an error of residual capacity based on an estimate value of an open circuit voltage in a normal state is increased.
Currently, a hybrid scheme that combines the Coulomb counting method and the OCV measurement method is the mainstream, but it is difficult to exclude an accumulated error due to current integration by simply combining both techniques. In particular, the calculation precision of residual capacity may be lowered or a calculation value of residual capacity may be rapidly changed in a state where discharging is continued like a hybrid vehicle, and it is difficult to secure stable precision.